Fabric in particular made of carbon yarns having low thickness variability combined with a specific basis weight range

ABSTRACT

Fabric having warp and weft yarns where the filaments in the yarns freely move relatively to each other. The basis weight of the fabric is related to the thickness standard deviation of a stack of three identical fabrics deposited on each other. For basis weights greater than or equal to 40 g/m 2  and less than 100 g/m 2 , the thickness standard deviation is less than or equal to 35 μm. For basis weights greater than or equal to 100 g/m 2  and less than or equal to 160 g/m 2 , the thickness standard deviation is less than or equal to 50 μm. For basis weights greater than 160 g/m 2  and less than or equal to 200 g/m 2 , the thickness standard deviation is less than or equal to 60 μm. For basis weights greater than 200 g/m 2  and less than or equal to 400 g/m 2 , the thickness standard deviation is less than or equal to 90 μm.

The present invention relates to the technical field of machinesallowing homogenization of the thickness of fibrous sheets and/orspreading of such fibrous sheets, in order to obtain lower basisweights. In particular, the invention relates to a method and to amachine allowing homogenization of the thickness of such sheets, as wellas to fabrics which may be obtained by applying such a method.

In the field of composite materials, the applicant was interested inproposing textile fabric sheets having a thickness as homogenous aspossible, so as to obtain parts with controlled final mechanicalproperties. In the case of fabrics, conventionally consisting of aninterlacing of warp yarns and of weft yarns, the latter is particularlydifficult.

The reinforcements for a composite are exclusively used with addition ofresin with different methods. The geometry of the final composite parttherefore directly results from the thicknesses of the reinforcementused. It is then clear that the use of thinner reinforcements willprovide lighter composite parts and also more performing since they havetheir fibres better oriented with less ripples. A fact which is lessobvious but also true is that these reinforcements, being also used in asometimes significant stack, it is necessary to reduce to a minimumtheir variations in thickness in order to make the geometry of theobtained composite part more reliable and robust. As the individualvariabilities of the folds will gradually add up, a great variability inthickness of the reinforcement will inevitably cause a strongvariability in thickness in the final part during the use of methodssuch as vacuum infusion.

Various documents are interested in spreading of fabrics, withouthowever mentioning the impact which may have the spreading applied onthe thickness and in particular on the thickness deviations which havethe obtained spread textile sheets. Mention may be made of documentsU.S. Pat. No. 4,932,107, U.S. Pat. No. 5,732,748, EP 670 921,WO2005/095689 and WO 94/12708. It is important to note that a tissuedoes not leave a weaving machine with homogenous thickness and opennessfactor on its width. Indeed, the actual principle of weaving induces ashrinkage phenomenon well known to one skilled in the art. Thisshrinkage is a reduction in the width of the warp sheet before and afterweaving. It is due to the interlacing action of the warp and weft yarns.The latter cover a shorter final distance because of their ripples overand under the warp yarns. The result of this is a reduction in the widthof the sheet upon leaving the comb of a weaving machine. As thisshrinkage is related to the ripples of the weft yarns, it is nothomogenous over the width of the fabric by the fact that the weft yarnsare more free, close to the edges and less held by less numerousneighbouring warp yarns. As they are less blocked and more free, theseedge of yarns therefore ripple more, the result of this is then a largerthickness and generally a larger openness factor. The thicknessdifference between the edges and the medium increases with the basisweight of the fabric.

It should also be noted that the over-thickness phenomenon of the edgesis very locally enhanced by the use of generally thermoplastic selvageyarns used on the edges of the fabric for blocking the last warp yarns.

All the fabrics proposed in the prior art, which are spread out aftertheir weaving, because of the applied spreading technique necessarilyhave significant thickness variation. In particular, in document U.S.Pat. No. 4,932,107, no mention of any width of the fabric, of theaverage width of the warp and weft yarns after spreading and ofhomogeneity of the openness factor on the fabric. Now, all theseelements determine the more or less homogenous thickness of the fabricobtained after spreading. If the examples proposed in this patent areconsidered, if a tension of 200 g/cm is applied on a fabric with a widthof 1.5 m, the value of the tension on the roller will be 150×200=30,000i.e. 30,000 g. This value is sufficient for generating flexure of therollers preventing the obtaining of a parallelism between the axes ofthe rollers and therefore a homogeneous pressure on the fabric, becauseof a higher pressure on the edges. There results a limitation of thewidth of the fabric to be processed in connection with the diameter ofthe rollers and of their length. In order to attempt to circumvent thisdifficulty, an increase in the diameter of the rollers may becontemplated for limiting flexure, but in this case, the inertia of thelatter will then become significant and the energy required forobtaining the amplitude and the frequency will increase in proportion.Moreover, it may be noted that patent U.S. Pat. No. 4,932,107 applied inits example 3B, 2 rollers with the diameter of 125 mm with a singleupper vibrating roller with a diameter of 60 mm, which on the one handdoes not give the possibility of obtaining satisfactory spreading and onthe other hand homogenization of the thickness. In a more general way,all the techniques for spreading fabrics described in the prior art donot give the possibility of adapting to the initial differences inthickness which the fabric has and therefore do not give the possibilityof obtaining satisfactory spreading and homogenization of the thickness.

There also exist fabrics made in two steps, the first step being theformation of sheets with low basis weight consolidated via a polymericbinder, and then producing the interlacing for forming a fabric. Suchfabrics because of the preliminary consolidation of the sheets providelesser possibilities in terms of deformability during theirapplications. Further, the polymeric binders used may not be compatiblewith the sheet of requirements under hygrothermal stress of the finalcomposite part.

In a more general context, mention may be made of documents US2007/066171 and US 2004/142618 which describe fabrics of reinforcingyarns, in dry form, without any data being provided on their thicknessvariation, which as indicated earlier is implicitly important, takinginto account the available methods for making such fabrics.

In this context, the invention proposes to react to the problemsmentioned above and encountered in the prior art and to provide a novelmethod and a novel machine giving the possibility of simply controllingthe thickness of the obtained textile sheet following a spreadingoperation, so as to obtain a low thickness variability, and this even onlarge widths of sheet.

In this context, the invention describes a method for spreading atextile sheet including at least warp yarns, according to which:

the sheet is caused to run between at least two rotary rollers, the axesof which extend parallel with each other and are substantiallyperpendicular to the running direction of the sheet,

the sheet is passed under pressure between at least one pressuregenerator for the rollers driven into axial oscillation and opposed inphase.

According to the invention, a pressure generator for the rollers isproduced with adjustable pressure values along said generator forspreading the sheet with low thickness variability.

Within the scope of the invention, it is also possible to ensure theapplication of a uniform pressure on the sheet so as to obtain a uniformthickness regardless of the width of the sheet. The rollers thusmodulate the applied pressure between the centre and the ends of thesheet, by taking into account the different thicknesses of the sheet soas to apply a uniform pressure on the material along the pressuregenerator. Typically, the pressure applied at the centre of the sheet isgreater than that applied on its edges so as to take into account theupper thickness of the sheet on its edges with respect to its centralportion.

According to a preferred embodiment, one of the rollers is made to beflexible and the other one rigid and localized supports distributedalong the axis of the roller are exerted on this flexible roller,substantially perpendicularly to its axis and with adjustable values forproducing the generator with adjustable pressure values. The flexibleroller may thus position itself automatically without any stress andthereby modulate the pressure applied on the sheet. In this case,preferably, the method inter alia consists of adjusting the position ofthe localized supports along the axis of the flexible roller and/ordistributing the localized supports regularly along the axis of theflexible roller.

According to a preferred embodiment which may be combined with theprevious one, the method inter alia consists of distributing thelocalized supports at most over the whole width of the textile sheet.

According to another preferred embodiment which may be combined with theprevious ones, the method inter alia consists of causing the textilesheet to pass over the periphery of the flexible roller between twopressure generators with adjustable localized pressure values of bothrigid rollers synchronously driven in rotation and in oscillation. Inthis case, preferably, the method consists of causing the textile sheetto pass between ⅙ and ⅓ of the periphery of the flexible roller. It isthus possible to do without the applied tension on the running textilesheet. Further, this facilitates obtaining an adjustable pressure on thetextile sheet, all along both pressure generators, between the textilesheet and the rigid rollers, given that this method for the passing ofthe textile sheet which no longer covers the rollers, as in patent U.S.Pat. No. 4,932,107, thus allows addition of a series of rigid supportsto both rigid rollers thereby avoiding any flexure of the latter. On theother hand, this passing method also facilitates the positioning of thelocalized supports on the flexible roller.

According to another preferred embodiment which may be combined with thepreceding ones, the method comprises the heating of the textile sheetduring its passing between the pressure generator(s).

According to another preferred embodiment which may be combined with theprevious ones, the method consists of bringing as a textile sheet, afabric including warp yarns and weft yarns each consisting of a set offilaments which may freely move relatively to each other within saidyarn, the spreading being produced on the warp yarns and on the weftyarns.

The present invention also describes a machine for spreading a textilefabric consisting of at least warp yarns, including:

at least two rotary rollers, the axes of which extend parallel with eachother and perpendicularly to a pressure generator, delimited betweenboth rollers,

a rotation motor-drive for at least one roller,

and a system for driving the rollers in axial oscillation with phaseopposition.

According to the invention, the machine includes a system for producingthe pressure generator with adjustable pressure values distributed alongsaid generator, for spreading the textile fabric with low thicknessvariability.

The machine, according to the invention, comprises either one, or evenall the features below when they do not exclude one from the other:

the system for producing the pressure generator includes from amongrotary rollers, a flexible roller and a series of localized supportswith adjustable pressure, distributed along the axis of the flexibleroller and acting on the flexible roller supported by at least one rigidroller,

the localized supports are equipped with a device for adjusting theirposition along the axis of the flexible roller,

the localized supports exert their pressure on the flexible roller, viarolling members with axial displacement,

the flexible roller delimits with two rigid rollers, the axes of whichextend parallel with each other, two pressure generators with adjustablelocalized pressure values, both of these generators being separatedbetween ⅙ and ⅓ of the periphery of the flexible roller,

the rollers have a diameter comprised between 30 mm and 60 mm,

the machine includes for each rigid roller, a series of rigid supportseach including a cradle attached to a chassis and having two supportingbranches each equipped with a rolling member for a rigid roller, havinga rotary movement and a translational movement along the axis of therigid rollers,

the system for driving the rollers into axial oscillation and in phaseopposition includes a motor synchronously driving by means of atransmission, two camshafts shifted by 180°, one of which acts on one ofthe ends of the flexible roller and the other one acts on one of theends of the rigid roller(s), the other end of the rollers being urged byan elastic system; this gives the possibility of ensuring perfectcontrol of the amplitude and of the operation, in phase oppositionbetween the flexible roller and both rigid rollers,

the machine includes a system for lifting the flexible roller, the endsof which are provided with plates on which acts the elastic system andon the other one of which acts the camshaft,

the machine includes a system for heating the textile sheet upon passingthe textile sheet between the pressure generators.

Such a method and such a machine make it thus possible to access thefabrics, object of the invention.

Actually, the object of the invention is fabrics consisting of warpyarns and of weft yarns, having a low thickness variation, characterizedby either one of the combinations of the following characteristics:

basis weight greater than or equal to 40 g/m² and less than 100 g/m² anda thickness standard deviation measured on a stack of three identicalfabrics deposited on each other and along the same direction which isless than or equal to 35 μm,

a basis weight greater than or equal to 100 g/m² and less than or equalto 160g/m² and a thickness standard deviation measured on a stack ofthree identical fabrics deposited on each other and along the samedirection which is less than or equal to 50 μm,

a basis weight greater than 160 g/m² and less than or equal to 200 g/m²and a thickness standard deviation measured on a stack of threeidentical fabrics deposited on each other and along the same directionwhich is less than or equal to 60 μm, or

a basis weight greater than 200 g/m² and less than or equal to 400 g/m²and a thickness standard deviation measured on a stack of threeidentical fabrics deposited on each other and along the same directionwhich is less than or equal to 90 μm.

In the fabrics according to the invention, the warp yarns and/or theweft yarns consist of a set of filaments, said filaments may freely moverelatively to each other within a same yarn. This is why the fabricsaccording to the invention may be obtained by means of the methodaccording to the invention. Unlike prior techniques, the methodaccording to the invention provides access to such fabrics having such acombination of features. Obtaining such fabrics with a width of at least100 cm, notably with a width from 100 to 200 cm, is possible. Thefabrics according to the invention may therefore have a great width anda very great length, for example approximately equivalent to the lengthof the available yarns, i.e. several hundred or thousands of meters.

The fabrics proposed within the scope of the invention, because of theirlower thickness variability, will give composite parts with a bettercontrolled geometry and will lead to a more robust global manufacturingmethod.

By thickness standard deviation, is meant the quadratic average of thedeviations to the mean, i.e.:

$\sqrt{\frac{1}{n}{\sum\limits_{i}\left( {x_{i} - \overset{\_}{x}} \right)^{2}}}$

-   with:-   n=number of values of measurements of the thickness of the stack of    three identical fabrics and oriented in the same direction, i.e. the    warp yarns on the one hand, and the weft yarns on the other hand are    oriented in the same direction within the stack,-   xi=a measurement value of the thickness of the stack of the three    identical fabrics,-   x=arithmetic mean of the thickness measurements of the stack of    three identical fabrics.

As the measured fabric unit folds become so thin, it appeared to be morerepresentative to measure the thickness standard deviation on a stack of3 folds.

Within the scope of the invention, the standard deviation may beobtained on a stack of three folds of a same fabric deposited on eachother and oriented in the same direction and placed under a pressure of972 mbars +/−3mbars, and notably from 25 one-off thickness measurementsdistributed over a surface of 305×305 mm, with for example one of thesides of the square which extends parallel to the warp yarns of thefabric. The method described in the examples may be used

Advantageously, the fabrics defined within the scope of the inventionconsist of warp yarns identical with each other and weft yarns identicalwith each other, and preferably warp yarns and weft yarns which are allidentical. In particular, the fabrics defined within the scope of theinvention consist of, preferably by at least 99% by mass, or evenexclusively consist of multi-filament reinforcement yarns, notablyglass, carbon or aramide yarns, carbon yarns being preferred. Asexamples of fabrics according to the invention, mention may be made ofthose having an architecture of the web type otherwise called taffeta,twill, a basket weave, or satin.

In particular, the invention relates to:

fabrics which have a basis weight greater than or equal to 40 g/m² andless than 100 g/m², a thickness standard deviation measured on a stackof three identical fabrics deposited on each other and along the samedirection which is less than or equal to 35 μm and an average opennessfactor from 0 to 1%. Advantageously, such fabrics have a variability ofopenness factor from 0 to 1%. Within the scope of the invention, theobtained spreading gives the possibility of obtaining such fabrics withyarns, and in particular carbon yarns, having a titer from 200 to 3,500Tex, and preferably from 200 to 800 Tex,

fabrics which have a basis weight greater than or equal to 100 g/m² andless than or equal to 160 g/m², a thickness standard deviation measuredon a stack of three identical fabrics deposited on each other and alongthe same direction which is less than or equal to 50 μm and an averageopenness factor from 0 to 0.5%. Advantageously, such fabrics have avariability of openness factor of at most 0.5%. Within the scope of theinvention, the obtained spreading gives the possibility of obtainingsuch fabrics with yarns, and in particular carbon yarns, having a titerfrom 200 to 3,500 Tex, and preferably from 400 to 1,700 Tex,

fabrics which have a basis weight greater than 160 g/m² and less than orequal to 200 g/m², a thickness standard deviation measured on a stack ofthree identical fabrics deposited on each other and along the samedirection which is less than or equal to 60 μm and an average opennessfactor from 0 to 0.5%. Advantageously, such fabrics have a variabilityof openness factor of at most 0.5%. Within the scope of the invention,the obtained spreading gives the possibility of obtaining such fabricswith yarns, and in particular carbon yarns, having a titer from 200 to3,500 Tex, and preferably from 400 to 1,700 Tex,

fabrics which have a basis weight greater than 200 g/m² and less than orequal to 400 g/m², a thickness standard deviation measured on a stack ofthree identical fabrics deposited on each other and along the samedirection which is less than or equal to 90 μm and an average opennessfactor from 0 to 0.1%. Advantageously, such fabrics have an opennessfactor variability of at most 0.1%. Within the scope of the invention,the obtained spreading gives the possibility of obtaining such fabricswith yarns, and in particular carbon yarns, having a titer from 200 to3,500 Tex and preferably from 800 to 1,700 Tex.

The openness factor may be defined as the ratio between the surface areanot occupied by the material and the observed total surface area, theobservation of which may be made from the top of the fabric with anillumination from below the latter. The openness factor (OF) isexpressed in percentages. For example it may be measured according tothe method described in the examples.

By openness factor variability, is meant the maximum difference inabsolute value obtained between a measured openness factor and theaverage openness factor. The variability is therefore expressed in %like the openness factor.

The average openness factor may be obtained, for example from 60openness factor measurements distributed over a surface of 305×915 mm offabric. The distribution may, for example, be achieved, by distributing⅓ of the openness factor measurements over a first third of the width ofthe fabric, ⅓ of the openness factor measurements on the second third ofthe fabric width corresponding to its central portion and ⅓ of theopenness factor measurements on the third portion of the fabric width.

By average openness factor, is meant the arithmetic mean of the 60measured openness factor (OF) values.

Mean openness factor=(OF1+OF2+OF3+ . . . +O60)/60

The detailed description which follows, with reference to the appendedFigures allows the invention to be better understood.

FIG. 1 is a schematic front view of a spreading machine according to theinvention.

FIG. 2 is a transverse sectional view of the spreading machineillustrated in FIG. 1.

FIG. 3 is a schematic front view of a spreading machine according to theinvention, in the raised position of the flexible roller.

FIGS. 4A and 4B are planar views of an example of a fabric illustratedbefore and after spreading, respectively.

FIG. 5 is a view giving the possibility of schematically illustratingthe spreading principle applied by the spreading machine according tothe invention.

FIGS. 1 to 3 schematically illustrate an exemplary embodiment of aspreading machine 1 according to the invention, adapted for spreadingwith a low thickness variability, a textile sheet 2 including at leastwarp yarns 3. Conventionally, by textile sheet, is meant a sheetmaterial consisting of yarns and by warp yarns, yarns extending alongthe running axis of the sheet on the machine. The textile sheets may beone-directional or fabrics. In the example illustrated in FIGS. 4A and4B, the sheet 2 is a fabric including warp yarns 3 and weft yarns 4,each warp 3 and weft 4 yarn consisting of a set of filaments t.According to a preferred embodiment, the spreading machine 1 accordingto the invention, is placed at the outlet of a weaving machine and atthe inlet of a system for winding up the sheet. It may also be providedthat the sheet to be spread out is from an unwinding system and which isnot directly positioned in line with a weaving machine.

The spreading machine 1 includes at least one first 5 and one second 6rotary rollers and in the illustrated example, a third rotary roller 7.The rotary rollers 5, 6 and 7 have axes A extending, parallel with eachother, and perpendicularly to the running direction f1 of the sheet 2 orperpendicularly to the warp yarns 3. The first roller 5 and the secondroller 6 delimit between them a first pressure generator G1 for thesheet 2 passing between the first and second rollers 5, 6. Also, in theexample illustrated in the drawings, the first roller 5 and the thirdroller 7 delimit between them a second pressure generator G2 for thesheet 2 passing between the first and third rollers 5, 7. Of course, thelength of the rollers is adapted to the width of the sheet 2 to bespread out, so as to have a greater length than the width of the sheet2. Typically, the length of the rollers is comprised between 1 m and 2m.

According to an advantageous feature of the invention, the rollers 5, 6and 7 are positioned in such a way that both pressure generators G1 andG2 are separated between ⅙ and ⅓ of the periphery of the first roller 5.In other words, the sheet 2 is in contact with the first roller 5exclusively between ⅙ and ⅓ of its periphery.

According to a preferred alternative embodiment, the second 6 and third7 rollers are positioned side by side in a horizontal plane, while thefirst roller 5 is positioned in the middle and above the second 6 andthird 7 roller.

The spreading machine 1 according to the invention also includes a motordrive 10 for ensuring synchronous driving into rotation around theiraxes A and along a same direction of rotation, second 6 and third 7rollers. In the illustrated example, the motor-drive 10 includes anelectric motor 11 controlled for synchronously controlling the speed ofrotation of the second 6 and third 7 rollers. The output shaft of theelectric motor 11 cooperates with a transmission belt 12 which drivesinto rotation pulleys 13 supported by shafts 14 mounted so as to beaxially secured to the first end of the second 6 and third 7 rollers.

In the illustrated example, the first roller 5 is not driven intorotation by the motor-drive 10. The first roller 5 is driven intorotation by the running force of the sheet 2 and by the rollers 6, 7. Ofcourse, it is possible to envision that the motor-drive 10 also drivesinto rotation the first roller 5.

The spreading machine 1 according to the invention also includes asystem 15 for driving the rollers 5, 6 and 7 into axial oscillation eachalong its axis A. More specifically, the driving system 15 allows axialoscillation of the first roller 5 in phase opposition with respect tothe second and third rollers 6 and 7 which are perfectly synchronized inaxial oscillation. In the example illustrated in the drawings, thedriving system 15 includes an electric motor 16 synchronously driving,by means of a transmission 17 such as a belt, first 19 and second 20camshafts giving the possibility of exerting an axial force on therollers. As this clearly emerges from FIG. 1, the cams of the camshafts19 and 20 are angularly shifted from each other by a value equal to180°.

The first camshaft 19 acts on the second end of the first roller 5 andmore specifically, on the transverse face of a shaft 21 axiallyextending from the first roller 5. According to an advantageousalternative embodiment, the first camshaft 19 acts on the shaft 21, viaa plate 21 a borne by the shaft 21. Thus, even when the first roller 5is moved vertically, the camshaft 19 continues to exert an axial forceon the shaft 21, as this will be explained in more detail in thecontinuation of the description.

The second camshaft 20 acts on the second end of the second roller 6 andin the illustrated example, of the third roller 7 also. According tothis illustrated alternative, the second and third rollers 6 and 7 areaxially equipped, at their second end, with shafts 22 in contact,through their transverse face, with the camshaft 20 which ensuressynchronized axial oscillation of the second and third rollers 6 and 7.Thus, the second and third rollers 6 and 7 have a perfectly synchronizedaxial oscillation.

The first ends of the first, second and third rollers 5, 6 and 7 areurged by an elastic system 25 which will compensate for the actionexerted by the camshafts 19, 20 on the second ends of the first, secondand third rollers 5, 6 and 7. In the illustrated exemplary embodiment,the elastic system 25 includes stacks of Belleville washers interposedbetween a support 28 on the one hand, and each shaft 14 and a shaft 29on the other hand extending axially from the first end of the firstroller 5. According to an advantageous alternative embodiment, a stackof Belleville spring washers 25 acts on the shaft 29 via a plate 29 aborne by the shaft 29. Thus, even when the first roller 5 is movedvertically, the stack of Belleville spring washers 25 continues to exertan axial force on the shaft 29 as this will be explained in more detailin the continuation of the description.

The driving system 15 as described above, gives the possibility ofensuring perfect control of the amplitude of operation in phaseopposition between the first roller 5 on the one hand and the second andthird rollers 6, 7 on the other hand. Moreover, this solution gives thepossibility of guaranteeing the desired movement of the rollers in spiteof wear phenomena due to suppression of the mechanical play betweencamshafts and the rollers.

Of course, the axial vibration frequency is adjustable, for example,from 5 to 50 Hz via the adjustment of the electric motor 16. Typically,the amplitude of the axial oscillation of the rollers is of the order of0.5 mm.

The spreading machine 1 also includes for the second and third rollers 6and 7, a series of rigid supports 31 giving the possibility ofsupporting without any flexure, the rollers while allowing theirmovements of rotation and oscillation. In the illustrated example, eachrigid support 31 includes a fork or a cradle 32 rigidly attached to achassis 33 preferably rigidly anchored to the ground. Each fork orcradle 32 thus has two supporting branches 34 each equipped with arolling member 35 for a roller 6, 7, which may both receive the movementof rotation and the movement of oscillation. In the example illustratedin FIG. 1, four rigid supports 31 support the rollers. Of course, thenumber of rigid supports 31 may be different notably depending on thelength of the rollers.

According to the invention, the spreading machine 1 includes a system 40for producing the first pressure generator G1 and in the illustratedexample also the second pressure generator G2, with adjustable pressurevalues distributed along the generator(s), for spreading the sheet 2with low thickness variability. In other words, the system 40 allowsmodulation of the pressure at will, along these pressure generators G1,G2 in order to apply uniform pressure on the sheet while taking intoaccount initial thickness differences of the sheet, with view tospreading the sheet with a low thickness variability.

According to a preferred embodiment, the system 40 includes as a firstroller 5, a flexible roller and a series of localized supports 42 withadjustable pressure, spread along the axis of the flexible roller 5 andacting on the flexible roller 5. As this more specifically emerges fromFIG. 2, the first roller 5 is mounted in a flexible way along its axis Ain the sense that it is free of any guiding bearing at both of its ends.

The flexible roller 5 may thus position itself automatically, withoutany stress, between the two other rollers 6 and 7. Conversely, thesecond and third rollers 6 and 7 are rigid since they are supportedwithout any flexure by the chassis 33. Each localized support 42 exertsits pressure on the flexible roller 5, via rolling members 43 with axialdisplacement. Thus, each localized support 42 is able to exert asubstantially vertical pressure force perpendicular to the axis of theflexible roller 5 while accepting the movement of rotation and axialoscillation of the flexible roller 5. For example, each localizedsupport 42 is a pressure actuator 44, the rod of which is equipped witha rolling member 43. Each pressure actuator 44 is connected to a controlunit not shown but known per se, allowing adjustment of the pressureexerted on the flexible roller 5. In the example illustrated in FIG. 1,the spreading machine 1 includes four pressure actuators. Of course, thenumber of pressure actuators 44 may be different.

According to an advantageous alternative embodiment, the localizedsupports 42 are equipped with a device 46 for adjusting their positionalong the axis of the flexible roller 5. Thus, the localized supports 42may be moved independently of each other along the axis of the flexibleroller 5 so as to be able to exert their pressure force in all theselected locations of the sheet 2. In the illustrated example, theactuators 44 are slidably mounted along a gantry 45 overhanging from adistance the flexible roller 5. Each actuator 44 is placed in a fixedposition by means of a system for locking the body of the actuator onthe frame, not shown, but of all types known per se.

According to an advantageous alternative embodiment, the spreadingmachine 1 according to the invention includes a system 48 for raisingthe flexible roller 5 in order to allow operations for placing the sheet2 between the flexible roller 5 and the rigid rollers 6, 7. In theillustrated example, the raising system 48 includes two actuators 49attached through their bodies onto the gantry 45 and the rods 49 a ofwhich act on the shafts 21 and 29 extending from both ends of theflexible roller 5. It should be noted that the elastic system 25 acts onthe shaft 29 of the flexible roller 5 while the camshaft 19 continues toexert an axial force on the shaft 21, even during operations for raisingthe flexible roller 5 because of the presence of the end plates 21 a and29 a, as illustrated in FIG. 3.

According to an advantageous embodiment characteristic, the spreadingmachine according to the invention includes a system 51 for heating thesheet and the rollers during the passing of the sheet between thepressure generators. The heating system 51 includes a nozzle 52 forsupplying the hot air produced by a hot air production unit not shownbut known per se. This supply nozzle 52 opens between both rigid rollers6 and 7 by directing the hot air flow towards the flexible roller 5along its portion located between both pressure generators G1 and G2.Typically, a heating unit of the Leister type is used for ensuringheating of the sheet 2 and of the rollers up to a temperature of 80° C.

In the foregoing description, the spreading machine 1 includes aflexible roller 5 and two rigid rollers 6, 7 defining two pressuregenerators G1, G2. Of course, the spreading machine 1 according to theinvention may have a similar operation by applying a single rigid roller6 defining with the flexible roller 5, a single pressure generator G1.Moreover, the spreading machine 1 described above, includes as localizedsupports 42, actuators exerting a pressure force on the flexible roller5. Other solutions may be contemplated with view to producing pressuregenerators with adjustable pressure values.

The spreading machine 1 according to the invention is particularlyadapted for spreading warp yarns 3 and also weft yarns 4 when the sheet2 is a fabric.

The application of a spreading method directly results from theforegoing description.

According to the method for spreading a sheet 2:

the sheet 2 is caused to run between at least two rotary rollers 5, 6-7,the axes A of which extend parallel with each other and aresubstantially perpendicular to the running direction of the sheet,

the sheet under pressure is passed between at least one pressuregenerator G1 of the rollers driven into axial oscillation and in phasedopposition,

and at least one pressure generator G1 of the rollers 5, 6-7 is producedwith adjustable pressure values along said generator so as to spread thesheet 2 with a low thickness variability.

It should be understood that it is thus possible to modulate thepressure between the centre and the edges of the sheet 2 so that theflexible roller 5 applies a uniform pressure on the sheet 2 taking intoaccount the thickness differences of the sheet. Of course, it may becontemplated that the pressures be identical along the contactgenerator.

During this spreading operation, the sheet 2 is maintained under tensionwith a substantially constant small value, by means of suitable systemsfor tensioning the sheet 2, located on its travel, upstream anddownstream, from the pressure rollers and designed for compensating theforces which may for example appear upstream, at the outlet of theweaving machine and downstream, at the winder of the sheet.

According to a preferred alternative embodiment, one of the rollers 5 ismade flexible and the other one 6-7 made rigid and, localized supports42 distributed along the axis of the roller and with adjustable valuesare exerted on this flexible roller, substantially perpendicularly toits axis in order to produce the generator with adjustable pressurevalue. Thus, different pressure values are exerted in differentlocations of the pressure generator in order to ensure proper spreadingof the yarns of the sheet 2.

According to an advantageous feature of the invention, the methodconsists of adjusting the position of the localized supports 42 alongthe axis of the flexible roller so as to selectively choose thelocations where the pressures are to be applied. For example, it ispossible to distribute the localized supports 42 in a regular way alongthe axis of the flexible roller. However, the adjustment consists ofdistributing the localized supports 42 at most over the whole width ofthe sheet 2. Indeed, regardless of the length of the sheet, thelocalized supports 42 should always act inside the delimited areaoverhanging the width of the sheet 2. In other words, the localizedsupports 42 should not act on an area of the flexible roller which isnever in contact with the sheet 2. According to a preferred exemplaryembodiment, the position of the actuators which are close to the edgesof the sheet are positioned so as to be at a distance of at least 50 mmfrom these edges. Typically, the actuators which are close to the edgesof the sheet are positioned so as to be at a distance of 150 mm fromthese edges. The actuators located between both of these actuators closeto the edges are positioned so that all the actuators are regularlyspaced apart. For example, the number of actuators is selected so thatthe distance between two neighbouring actuators is of at least 300 mm.According to a preferred embodiment alternative, the sheet 2 is causedto pass over the periphery of the flexible roller 5 between two pressuregenerators G1, G2 with adjustable localized pressure values. Both ofthese generators are delimited between the flexible roller 5 and twodriven rigid rollers 6, 7, synchronously, in rotation and inoscillation. Advantageously, the sheet 2 is caused to pass over theflexible roller 5, between ⅙ and ⅓ of the periphery of the flexibleroller 5.

According to a feature of the invention, the sheet 2 and the rollers areheated during its passing between the pressure generator(s).

It emerges from the foregoing description that the invention gives thepossibility of spreading the warp yarns of a one-directional sheet ofwarp yarns or interlaced warp yarns and/or weft yarns of a fabric. Thespread out textile sheets will, at least, partly be formed ofreinforcing fibres of the carbon, glass or aramide type whichconventionally consists of a set of filaments extending along thedirection of the yarn.

Advantageously, within the scope of the invention, the textile sheet tobe spread out will either exclusively consist of a one-directional sheetof warp yarns, or a fabric consisting of interlacing of warp yarns andweft yarns. Of course, in every case, the yarns are not secured to eachother by any binder or mechanical binding method of the sewing orknitting type which would hamper their displacement relatively to eachother and would not allow them to be spread out. In the case of afabric, the warp yarns and the weft yarns are only held together by theweaving. In particular, in the case of a textile sheet consisting of aone-directional sheet of warp yarns, the latter will consist of carbon,glass or aramide yarns. In the case of a fabric consisting of aninterlacing of warp yarns and weft yarns, it is either possible tospread out the weft yarns exclusively which, in this case, will beinterlaced with yarns playing the role of a support such as yarns in athermoplastic material, or to spread out both the warp yarns and theweft yarns. In every case, the yarns intended to be spread out in themethod according to the invention consist of a set of filaments whichmay freely move relatively to each other, and in particular of carbonyarns. Such yarns may initially have a circular section or preferablyrectangular section but at the outlet of the method according to theinvention, they will have a rectangular section following theapplication of pressure forces. In order to allow their spreading out,the yarns to be spread out and therefore the constitutive yarns of thefabrics according to the invention, will neither be impregnated, norcoated, nor associated with any polymeric binder which would hamper freedisplacement of the filaments relatively to each other. The yarns to bespread out are nevertheless most often characterized by a mass standardsizing level which may represent at most 2% of their mass.

A carbon yarn consists of a set of filaments and generally includes from1,000 to 80,000 filaments, advantageously from 12,000 to 24,000filaments. More preferably, within the scope of the invention, carbonfibres of 1 to 24K, for example, 3K, 6K, 12K or 24K, and preferentially12 and 24K are used The carbon yarns present within one-directionalsheets, have a titer of 60 to 3,800 Tex, and preferentially from 400 to900 tex. The one-directional sheet may be produced with any type ofcarbon yarns, for example high resistance (HR) yarns for which thetensile modulus is comprised between 220 and 241 GPa and the tensilebreaking stress of which is comprised between 3,450 and 4,830 MPa, yarnsof intermediate modulus (IM) for which the tensile modulus is comprisedbetween 290 and 297 GPa and the tensile breaking stress of which iscomprised between 3,450 and 6,200 MPa and high modulus (HM) yarns, forwhich the tensile modulus is comprised between 345 and 448 GPa and forwhich the tensile breaking stress is comprised between 3,450 and 5,520Pa (according to the “ASM Handbook”, ISBN 0-87170-703-9, ASMInternational 2001).

FIG. 4A schematically shows a fabric before its spreading out consistingof an interlacing of warp yarns and weft yarns with a slightly differentwidth because of the weaving. These may notably be 3K carbon yarns. Eachof the warp yarns and weft yarns consist of a set of filaments.Initially, the openness factor of the textile fabric is 4%.

FIG. 4B illustrates the fabric obtained after applying the spreadingmethod according to the invention. This fabric has an OF level of 0% andwarp and weft yarns of different width.

Within the scope of the invention, it is possible that the textile sheetbefore being subject to the method according to the invention has a zeroor non-zero openness factor. When initially the openness factor isnon-zero, applying the method according to the invention causes areduction of the openness factor which accompanies the obtaining ofhomogenization of the thickness of the textile sheet. Whether initiallythe openness factor is zero or non-zero, applying the method accordingto the invention causes a reduction in the thickness of the fabric byhomogenization of the thickness of the yarns making it up.

The invention is not limited to the described and illustrated examplessince diverse modifications may be provided thereto without departingfrom its scope.

Examples of carbon yarn fabrics obtained by means of the methodaccording to the invention are described in the examples hereafter.

Measurement Methods Used

Measurements of the Thicknesses

I. The Following Equipment is Used:

-   -   Vacuum pump from Leybold systems vacuum pump with reference        501902    -   Three-dimensional machine Tesa “micro-hite DCC 3D”    -   A glazed plate in toughened glass, with a thickness of 8 mm    -   A vacuum cover film with ref. 818260F 205° C. Nylon 6, green        from the supplier Umeco, Aerovac.    -   Bidim. AB1060HA 380 gsm 200° C. polyester, non-compressed rated        thickness 6 mm, supplier Umeco Aerovac.    -   PC with the software PC-Dmis V42    -   A ball sensor ø3 with a maximum trigger of 0.06N    -   A cutting wheel of the Robuso type    -   A cutting template 305×305 mm    -   Connection for a vacuum pump    -   A vacuum gasket SM5130 from the supplier Umeco Aerovac.

II. Description of the Measurement

-   -   Put the glass plate with the stack of three pieces of a same        fabric, as well as the environment, in the order from bottom to        top:        -   a bidim (a felt known to one skilled in the art)        -   stack of fabrics in the same direction, with the warp yarns            extending in the direction parallel to an edge of the square            of 305×305 mm        -   vacuum cover        -   Check the vacuum level (a vacuum of less than 15 mbars).    -   Establish a pressure reduced by a minimum of 15 mbars in the        vacuum cover, so as to place the stack under a pressure of 972        mbars +/−3 mbars.    -   Dimensional stabilization of the stack of fabrics under reduced        pressure has to be attained.    -   Leave the stack under this reduced pressure for at least 30        minutes before taking the points.    -   Take a physical point on the table in a manual mode (white point        on the top left of the table) by means of the joystick (joy on        the stick), validate and then switch to automatic mode (auto on        the stick):    -   Switch to automatic mode and wait till the measurement is made.

The program proceeds with taking 25 measurement points by means of itstriggering sensor.

The measurement of 25 <<blank>> points is repeated i.e. without thestack of the three fabrics in order to measure the thickness of thevacuum cover and of the glass.

Thus by a differential altitude measurement in between, with or withouta stack, we have a thickness average on 25 points, on the stack.

Openness Factor Measurements

The openness factors were measured according to the following method.

The device consists of a camera of the brand SONY (model SSC-DC58AP),equipped with a 10× objective and with a luminous table of the brandWaldmann, model W LP3 NR,101381 230V 50 Hz 2×15 W. The sample to bemeasured is laid on the luminous table, the camera is attached abracket, and positioned at 29 cm from the sample, then the sharpness isadjusted.

The measurement width is determined according to the textile fabric tobe analysed, by means of the ring (zoom), and of a ruler: 10 cm for opentextile fabrics (OF>2%), 1.17 cm for not very open textile sheets(OF<2%).

By means of the diaphragm and of a control photograph, the luminosity isadjusted so as to obtain an OF value corresponding to the one given onthe control photograph.

The contrast measurement software package Videomet, from Scion Image(Scion Corporation, USA) is used. After capturing the image, the latteris processed in the following way: by means of a tool, a maximum surfacearea is defined corresponding to the selected calibration, for example,for 10 cm-70 holes, and including an integer number of patterns. Anelementary surface in the textile sense of the term, i.e. a surfacewhich describes the geometry of the fabric by repetition is thenselected.

The light of the luminous table passing through the apertures of thefabric, the OF as a percentage is defined by a hundred multiplied by theratio between the white surface area divided by the total surface areaof the elementary pattern: 100* (white surface/elementary surface).

It should be noted that the adjustment of the luminosity is importantsince diffusion phenomena may modify the apparent size of the holes andtherefore the OF. An intermediate luminosity will be retained, so thatno too significant saturation or diffusion phenomenon is visible.

The fabrics with a width of 127 cm having basis weights, thicknessstandard deviations, openness factor, openness factor variability andshown in Table 2 below were able to be obtained by means of the methodaccording to the invention, by using the parameters as defined in Table1.

The machine used complies with FIGS. 1 and 2, with rollers of a diameterof 60 mm and a length of 1,700 mm, the actuators being spaced apart by320 mm, the two located at the ends being distant from the edge of thefabric by 155 mm. Table 1 gives as an example, for the fabrics shown inTable 2, the pressure force of the 4 pressure actuators 44 (No. 1 to 4)taken from one edge to the other of the fabric, with a running speed ofthe textile sheet (mm/min), a frequency (Hz) and a temperature (° C.).According to these exemplary embodiments, more significant forces areapplied in the central area of the fabric 2 allowing good spreading ofthe fabric 2, by compensating for the thickness difference existinginitially between the centre and the edges of the fabric, as illustratedin FIG. 5.

The AS4 3K yarns provided by Hexcel Corporation (Stamford USA) are highbreaking stress resistance yarns of 4,433 Mpa, of a tensile modulus of231 GPa having a titer of 200 Tex with filaments of 7.1 microns.

The AS4 12K yarns provided by Hexcel Corporation (Stamford USA) are highbreaking stress resistance yarns of 4,433 Mpa, of tensile modulus of 231GPa having a titer of 800 Tex with filaments of 7.1 microns.

The AS7 12K yarns provided by Hexcel Corporation (Stamford, USA) arehigh breaking stress resistance yarns of 4,830 Mpa, of tensile modulusof 241 GPa and having a titer of 800 Tex with filaments of 6.9 microns.

The IM7 6K yarns provided by Hexcel Corporation (Stamford, USA) areyarns with an intermediate breaking stress modulus of 5,310 Mpa, of atensile modulus of 276 Gpa and having a titer of 223 Tex with filamentsof 5.2 microns.

The IM7 12K yarns provided by Hexcel Corporation (Stamford, USA) areyarns with an intermediate breaking stress module of 5,670 Mpa, of atensile modulus of 276 Gpa and having a titer of 446 Tex with filamentsof 5.2 microns.

As an example, the tissue of 199 g/m² with AS4 3K before spreading hasan average openness factor of 10.5% (12.5% on the edges of the fabric,6.5% on the centre of the fabric) i.e. a variation of 6% of the opennessfactor between centre and edge, and an average thickness of 0.191 mm(0.201 mm on the edges of the fabric, 0.187 mm on the centre of thefabric) i.e. a 12% thickness variation between centre and edge. Thethickness standard deviation of the stack of three folds of thenon-spread fabric is 0.055 mm.

After spreading out, the openness factor of this same fabric passes to0.1% on average, i.e. a 99% reduction as compared with the non-spreadout fabric, with a maximum variation of 0.5% which moreover is not dueto an increase in the values on the edges, the average openness factorof the edges and of the centre being equal to 0.1%. A large portion ofthe measured openness factors are close to 0%, and a small populationabove 0.1% up to 0.5% in rare cases, inducing an average at 0.1% with amaximum variation of 0.5%. The thickness of the fabric after spreadingis 0.177 mm, i.e. reduced by 8% as compared with the non-spread fabric.The standard deviation of the stack of three folds of the spread fabricis 0.030 mm, i.e. a 45% gain as compared with the non-spread fabric.This information is gathered in Table 3 hereafter.

As an another example, a tissue of 75 g/m² in AS4C 3K will have anaverage openness factor before spreading of 45%, and an average opennessfactor after spreading of 0.8%, i.e. a 98% gain.

In every case, the application of the method according to the inventioncauses a significant reduction in the standard deviation of thethickness, of the average thickness, of the openness factor and of itsvariability. In particular, regardless of the basis weight of the fabricand the yarn used, by applying the method according to the invention,the gain in thickness standard deviation of 3 folds under the pressureof 972 mbars is equal at least to 20%, and in most cases is greater than30%.

TABLE 1 Warp and Weft Actuator pressure Density Yarn titer force (N)Speed Frequency Temperature yarns/cm Material designation Tex No. 1 No.2 No. 3 No. 4 mm/min Hz ° C.  75 g/m²-IM7 6K-Web 1.88 IM7GP 6K HSCP5000223 200 400 400 200 420 17 55  75 g/m²-AS4 3K-Web 1.88 AS4GP 3K HSCP5000200 200 400 400 200 420 17 55  98 g/m²-IM7 6K-Web 2.2 IM7GP 6K HSCP6000223 200 400 400 200 340 17 55  98 g/m²-AS4 3K-Web 2.45 AS4GP 3K HSCP5000200 200 400 400 200 340 17 55 160 g/m² IMA 12K-Web 1.79 IMAGS 12KHSCP6000 446 400 500 500 400 417 27 55 199 g/m² AS4 3K-Web 4.98 AS4GP 3KHSCP5000 200 200 400 400 200 500 17 55 199 g/m²-AS4 12K-Web 1.24 AS4GP12K HSCP3000 800 200 400 400 200 600 40 55 300 g/m²-AS7 12K-Twill 2/21.24 AS7GP 12K HSCP4000 800 200 400 400 200 600 40 55

TABLE 2 Thickness (mm) Aver- Standard Openness age deviation Aver-Factor (%) of the of the 3 age Vari- 3 fold fold thickness Aver- abil-stack stack per fold age ity  75 g/m² - IM7 6K - Web 0.169 0.023 0.0560.2 0.5  75 g/m² - AS4 3K - Web 0.145 0.028 0.048 0.8 0.8  98 g/m² - AS43K - Web 0.232 0.025 0.077 0.6 0.6  98 g/m² - IM7 6K - Web 0.222 0.0240.074 0.1 0.5 160 g/m² IMA 12K - 0.340 0.046 0.113 0.4 0.4 Web 199 g/m²AS4 3K - Web 0.531 0.030 0.177 0.1 0.5 199 g/m² - AS4 12K - 0.446 0.0380.149 0 0.1 Web 300 g/m² - AS7 12K - 0.742 0.078 0.247 0 0.1 Twill 2/2

TABLE 3 Thickness (mm) Measured average Standard deviation OpennessFactor (%) thickness per fold of the stack of Maximum on a stack ofthree folds three folds Average variability Before After Before AfterBefore After spreading spreading Gain spreading spreading Gain spreadingspreading Gain 199 g/m² AS4 3K-Web 0.191 0.177 8% 0.055 0.030 45% 10.50.1 99%

1- A fabric comprising of warp and weft yarns, wherein said fabric has abasis weight greater than or equal to 40 g/m² and less than 100 g/m² anda thickness standard deviation measured on a stack of three identicalfabrics deposited on each other and along the same direction which isless than or equal to 35 μm, or a basis weight greater than or equal to100 g/m² and less than or equal to 160 g/m² and a thickness standarddeviation measured on a stack of three identical fabrics deposited oneach other and along the same direction which is less than or equal to50 μm, or a basis weight greater than 160 g/m² and less than or equal to200 g/m² and a thickness standard deviation measured on a stack of threeidentical fabrics deposited on each other and along the same directionwhich is less than or equal to 60 μm, or a basis weight greater than 200g/m² and less than or equal to 400 g/m² and a thickness standarddeviation measured on a stack of three identical fabrics deposited oneach other and along the same direction which is less than or equal to90 μm, and wherein the warp yarns and/or the weft yarns consist of a setof filaments which may freely move relatively to the other filamentswithin said warp yarns and/or said weft yarns. 2- The fabric accordingto claim 1, wherein the warp yarns are identical with each other and theweft yarns are identical with each other. 3- The fabric according toclaim 1 wherein the warp yarns and/or the weft yarns consist of carbonfilaments. 4- The fabric according to claim 1 which has a basis weightgreater than or equal to 40 g/m² and less than 100 g/m², a thicknessstandard deviation measured on a stack of three identical fabricsdeposited on each other and along the same direction which is less thanor equal to 35 μm and an average openness factor from 0 to 1%. 5- Thefabric according to claim 4, which has an openness factor variability ofat most 1%. 6- The fabric according to claim 4 which comprises warpyarns and/or weft yarns having a titer from 200 to 3,500 Tex. 7- Thefabric according to claim 1 which has a basis weight greater than orequal to 100 g/m² and less than or equal to 160 g/m², a thicknessstandard deviation measured on a stack of three identical fabricsdeposited on each other and along the same direction which is less thanor equal to 50 μm and an average openness factor from 0 to 0.5%. 8- Thefabric according to claim 7, which has an openness factor variability ofat most 0.5%. 9- The fabric according to claim 1 which has a basisweight greater than 160 g/m² and less than or equal to 200 g/m², athickness standard deviation measured on a stack of three identicalfabrics deposited on each other and along the same direction which isless than or equal to 60 μm and an average openness factor from 0 to0.5%. 10- The fabric according to claim 9, which has an openness factorvariability of at most 0.5%. 11- The fabric according to claim 7 whichcomprises warp yarns and/or weft yarns having a titer from 200 to 3,500Tex. 12- The fabric according to claim 1 which has a basis weightgreater than 200 g/m² and less than or equal to 400 g/m², a thicknessstandard deviation measured on a stack of three identical fabricsdeposited on each other and along the same direction which is less thanor equal to 90 μm and an average openness factor from 0 to 0.1%. 13- Thefabric according to claim 12, which has an openness factor variabilityof at most 0.1%. 14- The fabric according to claim 12 which compriseswarp yarns and/or weft yarns having a titer from 200 to 3,500 Tex. 15-The fabric according to claim 4 wherein average openness factor and theopenness factor variability are measured by conducting 60 opennessfactor measurements distributed over a surface of 305×915 mm of fabric.16- The fabric according to claim 1 which has a width of from 100 to 200cm. 17- The fabric according to claim 1 wherein the thickness standarddeviation is measured on a stack of three identical fabrics deposited oneach other and oriented in the same direction and placed under apressure of 972 mbar +/−3 mbar by conducting 25 point-like measurementsdistributed over a surface of 305×305 mm. 18- The fabric according toclaim 1 wherein said fabric is in the form of a web, twill, basket weaveor satin type. 19- The fabric according to claim 1 wherein the warpyarns and weft yarn are neither impregnated, nor coated nor associatedwith any polymeric binder.